1. Field of the Invention
The present invention relates to highly densified silicon nitride sintered bodies having excellent mechanical strength and oxidation resistance and a method for producing the same.
2. Description of the Prior Art
Silicon nitride sintered bodies have excellent mechanical strength, heat resistance, thermal shock resistance and corrosion resistance as compared with metal materials and therefore it is intended to apply these bodies to various machine parts which are operated at high temperature at which metal materials cannot be used. The development of applications of these materials has progressed extensively.
Requisite properties for materials to be used as machine parts which operate at high temperature include, high oxidation resistance, which is required in view of the durability and dimensional stability in use of the parts for a long period of time, and mechanical properties, such as high strength at high temperature.
In general, when comparing the mechanical properties of silicon nitride sintered bodies with silicon carbide sintered bodies, silicon nitride sintered bodies have been superior to silicon carbide sintered bodies in view of their mechanical properties, for example, flexural strength, fracture toughness, thermal shock resistance and the like. On the other hand, silicon carbide sintered bodies are superior in view of the oxidation resistance to silicon nitride sintered bodies. Therefore, in any prior silicon nitride sintered bodies, the improvement of the oxidation resistance is strongly demanded together with the improvement of the mechanical properties.
Manufacturing processes for obtaining highly densified silicon nitride sintered bodies have included, pressureless sintering and hot press sintering, and the like, but the pressureless sintering can produce easily and cheaply various products in mass production but requires a large amount of sintering aid to achieve appropriate densification for high temperature strength and oxidation resistance. The hot press sintering and the like can obtain the sintered bodies having a high density by using a small amount of sintering. However, in order to obtain large articles or complicated shapes in mass production this process has a great drawback because the production cost is very high.